Low frequency amplifier



April 18, 1933. H. A. WHEELER LOW FREQUENCY AMPLIFIER Filed Feb. 17. 1927 k l i jl l gl l M a BY Z W ATTORNEYS Patented A r. 18,- 1933' UNITED STATES PATENT orrlca HAROLD ALDEK OF GREAT NECK, NEW YORK, ASSIGNOB TO CORPORATION, OF JERSEY CITY, NEW JERSEY, A. CORPORATION 01' DELAWARE LOW FREQUENCY AMZLIFIEB Application filed. February 17, 1927. Serial No. 168,925.

This invention relates to wave signalling apparatus and more particularly concerns a system of low-or audio-frequency amplification for use in connection with radio, telephone, and other signalling systems.

In the transmission and reception of radio communications such as speech and instrument or vocal music, it is customary to employ a vacuum tube of the well known threeelement type, and to amplify the audio-or low-frequency signal by means of an, audiofrequency ampli er comprising one or more vacuum tubes connected in some suitable manner. For example in one well known and highly efiicient typelof audio-frequency amplifier, the vacuum tubes are connected in cascade through transformers, the input energy being supplied from the detector-tube in a radio receiver, orfrom a microphone, in the case of a transmitter. An amplifier of this type can be used in a public address system, for amplifying the speech input of transmitters or in other systems of the type in which the amplification of low-frequency signals is desired. I

The audio-frequency amplifiers now in general use in radio receiving sets leave much to be desired'when amplification is required over the ranges of frequency and amplitude encountered in the loud speaker reproduction of voice, music and other sounds. The introduction of the cone. type loud speaker, combined with an increasingly exacting public demand for higher quality, often regardless of cost, makes it necessary to greatly improve the quality of reproduction of the present amplifiers.

In an audio-frequency interstage transformer of the type usually employed in audio-frequency amplifiers, the secondary winding is placed over the primary winding,

and is therefore removed a substantial distance from the transformer core. This construction gives rise to a rather high so-called leakage reactance in the secondary winding, which reactancewith the internal ca pacity comprises the equivalent of a'series connected inductance and capacity in parallel with this winding, and gives rise to a resonance peak in the voltage amplification curve of the transformer, which peak usually occurs at some frequenc between three thousand and ten thousand resonance peak or rise in amplification is objectionable, as it results in non-uniform amplification at the higher frequencies, and may also cause oscillations which are fed back and may be sustained throughout the amplifier as a-whole. I 4

Further distortion in audio-frequency amplifiers' arises from the fact that the grid of a vacuum tube draws a certain amount of energy from the input circuit of the tube,that

is, the secondary winding of the input transformer; and, under certain conditions, the maximum primary energy which can be supplied to the primary winding'of the input transformer by the late circuit of the preceding tube is not su cient to supply both the 1 required secondary or grid energy and the degree of grid voltag'e variation necessary for satisfactory operation of the tube under consideration. Distortions arising from this cause may be reduced by the use of a grid, or C, battery to put a negative-potential bias on the grid, but this expedient reduces the plate current obtainable from the tube for a given plate voltage, and is for this and other reasons undesirable.

In audio-frequency amplifiers the plate voltages for the various tubes are usually su lied b a single late, or B, battery. TIP: internzil resistan e of this battery has been found to increase ra idl with use, and long before the useful li e o the battery is at an end, this resistance reaches a value cycles per second. This which results inan intersta e feed-back of audio frequency voltages an consequent oscillations and distortion of the signals being amplified.

With the above and other considerations in mind, it is proposed in accordance with the present invention to provide a system of audio-frequency amplification embodying means for giving a comparatively large power output with a minimum amount of distortion, and, more specifically, it is pro-,

posed to provide a system of this type emodying certain devices, circuits and other expedients by means of which distortion and oscillation arising from the leakage reactance of the interstage transformers, the energy drawn by the vacuum tube. grid circuits, and the resistance of the plate orB battery, are efiectively reduced or eliminated.

Other specific objects, advantages and characteristic features of the invention will become apparent as the description thereof progresses.

In describing the invention in detail, reference will be made to the accompanying drawing, in which;

Fig. 1 is a diagrammatic representation of a two-stage audio-frequency amplifier embodying certain features of the present invention;

-Fig. 2 represents a two-stage audio-frequgncy amplifier of slightly modified form; an

Fig. 3 represents a three-stage audio-frequency amplifier constructed in accordance with the present invention.

Referring to the drawing and more particularly to Fig. 1, an amplifier embodying the present invention has been illustrated in connection with the detector audion tube V of a radio receiving set, the input energy for this tube being supplied from any suitable source such as a radio-frequency amplifier of any suitable type. The grid 1 of the detector tube V is connected to a source of radio-frequency current supplied through the wires 2 and 3 and the usual grid condenser C1, the filament 4 of this tube being connected with the same supply through wire 5; and a rid leak or high resistance R is connected etween the grid 1 and the filament 4 through a circuit which is obvious fromthe drawing. The output or plate circuit of the detector tube V is connected in series with the primary winding 6 of an audio-frequency interstage transformer T through a circuit whlch includes the plate 7 of the tube V, the wlres 8 and 9, primary winding 6, wires 10 and 11, the plate or B battery 12, wire 13, filament reslstance 14, and wires 15 and 16 to the filament 4 of the tube V.

A condenser C is preferably connected between the plate 7 and the filament 4 of the detector tube V as shown, this condenser actmg as a shunt for the radio-frequency voltages which may flow in this plate circult, and thereby prevent possible radio frequency feed-back due to these voltages. The condenser C is of a capacity such that its presence has no appreciable effect on the flow of the audio-frequency current in the plate circuit of the tube V. 4

The transformer T may take any suitable form, but is preferably of the iron core type, and preferably has a step-up voltage transformation ratio of about two-.to-one, al-

thoughother suitable ratios of voltage transformation may be employed. The secondary wlnding 17 of the transformer T is connected between'the grid 18 and the filament 19 of the first audio-frequency amplifying tube V this circuit includin a resistance R, connected in series therewlth as shown.

A condenser C is connected directly between the grid 18 and the filament 19 of the tube V The constants of the resistance R, and the condenser C as well as the purpose of the particular connection of these devices, will be hereinafter described.

The plate circuit of the tube V is connected in series with the primary winding 20 of a second audio-frequency transformer T this circuit being obvious from the drawing. The transformer T preferablyhas a voltage transformation ratio of one--to-one, and its windings are so arranged that the relative polarities of its adjacent terminals 21 and 22 are the same at any iven instant. This relation of relative p0 arity at the transformer terminals may be obtained by winding the primary coil 20 and secondary coil 23 in the same direction. -This partlcular arrangement comprises an important element of the present invention, as Wlll hereinafter explained. A condenser C, is connected ,between the plate terminal 21 of the primary winding 20 and the grid terminal 22 of the secondary winding 23 of the transformer T as shown.

The secondary winding 23 of the transformer T is connected between the grid 24 these filamens being controlled by the ad justable filament resistance 14. The plate voltage of the tubes V, V and V is supplied by a plate, or B,'battery 12 through obvious circuits. v In the operation of the audio-frequency amplifier illustrated in Fig. 1, the radio-frequency energy impressed between the grid 1 and filament 4 of the detector tube V is rectified, or detected, in the usual and well known manner, and results in a current having an audio-frequency component flowing in the plate circuit of this tube. As pointed out above, the condenser C2 connected between the plate 7 and the filament 4 of the tube V, eliminates or effectively reduces radio-frequency voltages in. this plate circuit,-and thereby prevents possible feed-back of such voltages to the grid circuits of the radiofrequency ampli ying tubes, tube V.

As explained above, the inherent leakage reactance of the secondary winding 17 of thetransformer T is very high, this being or the detector due to the disposition of thiswinding outside of the primary winding 6 and therefore removed a considerable distance from the transformer core. The circuit through the secondary winding 17 has a point of resonance due to this reactance, which point occurs at the higher audible frequencies, that is, from two thousand to ten thousand cycles per second. In order to eliminate this resonance peak or to reduce it to a negligible 'value,.the resistance R,, which may be the pass any appreciable amount of current be- 7 through the condenser C low a given frequency, about two thousand or three thousand cycles, but readily passes current at frequencies above three thousand cycles. When the frequency of the voltage in the secondary winding 17 of the transformer T is below a given value, say about two thousand cycles, energy is delivered to the grid 18 of the tube Vi in the usual manner. When the frequency of the secondary voltage increases to a point at which the increase in amplification due to the resonance peak of the secondary winding 17 is appreciable, a comparatively large current flows This circulating current through the condenser (1, reduces the voltage applied to the grid circuit of the tube V for two reasons. The flow, of current through the condenserC, causes an'increased current to fiowthrough the resis"- I ance R and therefore an appreciable voltage drop occurs across this resistance, which drop reduces the voltage applied to the grid circuit at these higher frequencies. Also, the increased current through the secondary winding 17,. due'to the current passed by I the condenser 0 increases the load on the transformer T and thereby reduces the secondary terminal voltage thereof in the well known manner. Thus at the higher frequencies, that is, frequencies above two thousand or three thousand cycles per second, at which frequencies the grid circuit voltage is inclined to increase to an undesirable peak due to the leakage reactance of the secondary winding 17-, the combination of resistance R, and condenser '0, act to reduce this grid voltage, with the result that the grid voltage remains substantially constant over the audiblelrange of frequencies, and no undesirable peak occurs in the. amplification curve.

As noted above, the grid current in the grid circuit of the first stage tube V is very small,

so that the effect of the resistance R, on the average grid voltage is negligible. In the grid circuitof the second stage tube V however, this is not the case. The interstage transformer T used in this amplifier preferably has a one-to-one ratio of voltage transformation, for reasons which will hereinafter appear, and due to the amplification obtained in the first stage, the current flowing in the" grid circuit of the tube V is large as com- 7 pared with that flowing in the grid circuit of the tube V For this and other reasons, it has been foundthat the combination of theresistance R and the condenser 0 as employed in the first audio-frequency stage is 30 not as satisfactory for use in the second stage, and other more eflicient means are employed to prevent distortion due to the leakage of the transformer T The condenser 6,, connected between the primary terminal 21 and the-secondary terminal 22 of the transformer T is employed Y to prevent distortion at the higher frequengies in the stage or stagessubsequent to the rst. explained in various ways. According to one theory, the condenser C together with the tube V form a shunt including a capacity 1 and resistance across the secondary winding The action of this condenser maybe 23 of the transformer T this shunt path starting at the terminal 22 and including the condenser C wire 27 the plate 28, and filament 19 of the tube V the-wires 29 and 30 to the secondary winding 23 of the transformer T The condenser G5 has a capacity such that at least the higher frequencies, that is, the frequencies at and above which the undesirable resonance peak occurs, are passed thereby, and hence a load is placed on the just traced, at these higher frequencies. This loadreduces the terminal voltage of the secondary winding 23 in the well known manner, and so counteracts or nullifies the effect of the resonance peak in a manner somewhat similar to that described above in connection with the grid circuit of the tube V The theory of operation of the condenser secondary winding 23 through the shunt path T had what is known as unity coupling, that o u a ratio at all frequencies, but has an Increase ed by the use of a transformer having a onein its voltage amplification ratio which cies, thereby effectively reducing or limiting the amplification peak referred to. Thus the condenser C effectively eliminates distortion arising from the leakage reactance of the secondary winding 23 in the last audio stage or stages. 4

As pointed out above, a further source of distortion arises from the fact that the grid circuit of the vacuum tube draws a certain amount of energy during its operation, and the input apparatus for supplying such grid circuit is sometimes incapable of supplying this energy and at the same time supplying the necessary degree of rid voltage variation required for the satisfactory operation of the tube. To explain this phenomenon more in detail; when the grid of a vacuum tube is supplied with a positive potential with respect to the filament thereof, a certain amount of current flows from the grid to the filament, due to the passage of electrons from the filament to the grid. This flow of current draws energy from the input transformer supplying the grid circuit, and if the current is comparatively large, the voltage supplied by the transformer is appreciably reduced, this reduction being due to the characteristic behavior of a loaded transformer; that is, the amount of energy which can be supplied to the primary winding of the transformer from the plate circuit of the preceding tube is limited by the plate resist ance. It should be noted that when a negative voltage is applied to the grid with respect to the filament, no current flows in the grid circuit, and, for this reason, the grid circuit voltage is not reduced. Hence the voltage reduction in the grid circuit occurs during one-half of the grid voltage cycle only,

and results in a very marked distortion of the slgnals. The grid current drawn by the tube during the positive half of the voltage cycle increases very rapidly as the grid voltage increases, and the power which canbe supplied from the preceding plate circuit decreases as the transformer ratio is increased above a certain value.

Distortion due to the grid current as described above is objectionable -onlv when comparatively large variation of grid voltage is used, and for this reason, very little, if any, objectionable distortion arises from this source in the first audio stage Where'the grid voltage variations are comparatively small.

In the amplifier of the present invention, distortion due to grid circuit current is avoid transformer T has a one-to-one voltage 7 transformation ratio. The use of a one-to-one ratio transformer gives undistorted amplification for several reasons. In a one-to-one ratio transformer, an increase 1n current in the secondary winding does not decrease the 7 secondary voltage to the extent that such voltage would be decreased by such current if a transformer having a higher ratio of voltage transformation were employed. A

comparatively small increase in secondary 5 current in a transformer of this ratio, such as the increase in current occurring when the voltage of the grid of a vacuum tube is positive with respect to its filament, does not ordinarily require an amount of power from the plate circuit of the preceding tube in excess of the amount that this plate circuit can supply. In other words, the ratio of the amount of power dissipated in the grid circuit of a given tube to the amount of power which can besupplied by the plate circuit of the preceding tube is much greater as the ratio of voltage transformation of the transformer connecting these tubes is increased, and for this reason the positive voltage to 1 which the grid of the tube under consideration can be raised is limited by the transformer ratio, and is greater for transformers having low ratios of transformation. Thus, b the use of the one-toone ratio transformer 2 in the amplifier of Fig. 1, the plate circuit of the tube V supplies sufiicient power through this-transformer to supply the current drawn by the grid circuit of the tube V; durin the half of each voltage cycle when the grid 24 is positive with respect to the filament 25, and at the same time supplies sufficient voltage to raise this grid 24 to the desired, positive potential.

Referring now more particularly to Figs. 2 and 3, two audio-frequency amplification systems have been disclosed in which distortions due to the leakage reactance of tl e secondaries of the transformers T T and -T have been effectively eliminated by the same means as those disclosed and described in connection with the amplifier of Fig. 1, namely the combined resistance and con-- denser iR, and C in the first stage, and the condenser C in the remaining stages of amplification. These amplifiers also include transformers T and T having one-to-one ratios of voltage transformation. in all but distortions or oscillations arising from the v and condenser C has been shown only in feed-back of audio-frequency voltages due to high resistance in the plate, or B, battery.

It is desirable from the standpoint of economy to employ a common plate, or B, battery for supplying the plate circuit voltages of all of the tubes of the amplifying apparatus. This battery is designated at 31 in Figs. 2 and 3; and the plate-filament circuits of all of the tubes V, V and V are connected in multiple with this battery as clear- 1y shown in the drawing. It has been found t at the internal resistance of a plate bat tery of the usual dry cell type increases very rapidly with use, and often reaches a very high value, such as one hundred ohms or more, before the useful life of the battery is at an end. Since the plate currents from all of the audio-frequency tubes, including the detector tube V, flow through the battery 31, this high internal resistance often given rise to a feed-back of audio-frequency voltages, which affects the amplification and may cause sustained oscillations in the amplifier as a whole. y

In order to avoid oscillation, distortion or impaired amplification arising from the high internal resistance of the plate battery 31,

each of the amplifiers of Figs. 2 and 3 is provided with a. filter circuit comprising an in ductive reactance coil L5 and a condenser C in the plate circuit of the detector tube V.

' The reactance coil L is preferably provided with an iron core and has an inductance such that the flow of audio-frequency currents therethrough is substantially prevented. The condenser G has a capacity such that audio-frequency alternating currents are readily passed thereby. In operation, the audio-frequency component of the plate current in the detector tube V is passed by the wires 32 and 33, the primary winding 34 of the transformer T wire 35, condenser G8,

and wires 36, 37 and 38 to the filament 39 of the tube V. The direct current component of the detector tube plate circuit cannot pass through the condenser 0 and as the coil L offers but little resistance to the flow of this direct current, its path is fromthe plate 40, through wires 32 and 33, primary winding 34, wires 35 and 41, coil L wires 42 and 43, plate battery 31, wire 44, filament resistance 45 and wires 46 and 47 to the filament 39 of the tube V. The audio-frequency currents .in the plate circuits of the tubes V and V Although the filter comprising the coil L8 the plate circuit of the detector tube V, a similar filter may be employed in the plate circuit of the tube V of the amplifier shown in Fig. 2 and in the plate circuits of the tubes V and V of the amplifier of Fig. 3, if desired; In this manner, feed-back from the plate circuit of the audio-frequency amplifying tube in the last stage to all of the preceding tubes may be prevented. Obvious- "ly, it is most important to prevent audio-fre- V V with a three-stage audio-frequency amplifier has been disclosed, this amplifier embodying the plate circuit filter L and C and the combined resistance and capacity it, and C, in the first, stage, and the'condenser C in the second and third stages, to prevent distortion due to the resonance amplification peak of the transformers T T and T The amplifier of Fig. 3 also employs transformcrs T and T having one-to-one ratios of voltage transformation in the second and third stages, thereby preventing distortion due to gridcircuit current as described above in connection with the amplifier of Fig. 1. The amplifier of Fig. 3 is desirable for use where greater amplification is desired than that which can be obtained by the use of the condenser C ,travers1ng a path 1nclud1ng the two stage amplifiers of Figs. 1 and 2.

In the amplifiers of the present invention, the constants of the various coils, condensers, transformers and batteries used may be varied Within certain limits, and the constants of these means which may advantageously be employed in'any given'amplifying circuit depend on the characteristics of the tubes used in such circuit, on the character of the power supply available and on various other factors. One set of constants which has been found to give excellent results will be given,

it being clearly understood that the values set forth represent only one specific embodiment of the invention disclosed,- and that many other suitable values of these constants may be employed within the scope of the invention. In the particular arrangement referred to, the capacities of the condensers are as follows C .0001 microfarads; C2,, .002 microfarads; C 1 microfarad; C .0001 microfarads; C .006 microfarads. The inductances of the various coils and windings are as follows: L 100 henries (5,000 ohms D.

C.) primary winding of the transformer T over the primary winding. The transformers T and T are also wound on figure 8 laminated magnetic cores and these transformers have a transformation ratio of one-to-one, their windings comprisingeach about 15,000 turns. The wire used in" all of the transformer windings is about No. 40 B. & S. The

choke coil L may comprise about 20,000

turns of No. 36 B. St S. wire, wound on a gure 8 core or a plain closed core. The values of the constants of the various devices given above, are particularly adaptable when vacuum tubes of the 201A type are employed.

The amplifying system of the present invention has many advantageous features. Distortions arising from the inherentleakage reactance of the transformers, and oscillations resulting from the amplification peak of the transformers are effectively prevented by the arrangement of the resistance R and the condensers C and C, which act to oppose or counteract the effects of this leakage reactance as described above. With the use of the one-to-one ratio transformers in the last audio-frequency stages, amplification of excellent quality and volume is obtained without the use of grid circuit, or C, batteries. The use of the filter circuit including the choke coil L and the condenser C prevents feed-back and oscillations due to plate, or B, battery resistance, and is also useful in preventing fluctuations in the plate circuit current when a generator or a rectifier is used to supply the'plate voltage. By reason of the combined action of the vari- Ous features enumerated with a minimum of sustained oscillations, and gives a very satisfactory volume as compared with the audiofrequencg amplifying systems previously V, cm loye lthough the present invention has been described in connection with certain rather the am lifier as a whole acts to amplify the signa s received istortion and without wherein the feature including the condenser C, was not employed, or the one-to-one ratio transformer with the condenser connected between its primary and secondary windings might be employed independently of the other features disclosed. Furthermore it will be obvious that the features introduced by the invention'may be advantageously employed in any low-frequency system, such as public address systems and speech amplifiers in transmitters, merely by substituting the systems herein described for those previously known.

I claim: 7

1. In a thermionic amplifier, the combination with a thermionic tube having input and output sections of an input transformer associated with said input section, resistance serially included in circuit with said transformer and said input section, and a capacity directly shunting said input secti0n,said resistance and capacity being so proportioned according to the leakage inductance and distributed ca acity of said transformer as to ensure a su stantia'lly uniform degree of amplification over a preselected range in in circuit with said secondary winding and input section, and a capacity directly shunting said input section, said resistance and capacity bemg proportioned on the basis of the leakage inductance and distributed capacity of said secondary windin to produce a substantially uniform ampli cation over a preselected frequency range.

3. In a thermionic amplifier, the combination with a thermionic tube having anode, cathode and grid, of an input transformer having its secondary winding connected between said'cathode and grid, and resistance serially included in circuit with said secondary winding and said grid and a capacity directly shunting said cathode and grid, said resistance and capacity being proportioned to offset the distorting effect on the amplified current at certain frequencies due to the leakage inductance and distributed capacity of said secondary winding and to ensure a substantially uniform degree of amplification over a preselected range in frequency.

on am lification at certain frequencies due to 8. In an audio-frequency amplifier, the

the lea age inductance and distributed capaccombination with a pair ofzthermionic tubes ity of said transformer and to ensure a sub stantially uniform degree of ampification within the rangeof audible frequencies.

5. ,In an audio-frequency amplifier,thecombination with a pair of thermionic tubes each having anode, cathode and grid, of a twowinding transformer coupling said tubes in cascade, with the primary winding terminal joined to the anode of the preceding tube having the same open-circuit polarity as the secondary winding terminal joined to the grid 'of the succeeding tube, a capacity extending from the anode of the preceding tube to the grid of the succeeding tube, and a conductive path extending between said cathodes, sald capacity being proportioned on the basis of the leakage inductance and distributed capacity' of said transformer to provide a substantially uniform degree of amplification within the range of audible frequencies.

6. In an audio-frequency amplifier, the combination with a pair of thermionic tubes each having anode, cathode and grid, of a two-winding transformer of substantially unity ratio, coupling said tubes in cascade, with the primary winding terminal joined to the anode of the preceding tube having the same open-circuit polarity as the secondary winding terminal joined to the grid of the succeeding tube, a capacity connecting the anode of the preceding tube with the grid of the succeeding tube, and a conductive path extending between said cathodes,- said capacity being selected on the basis of the leakage inductance and distributed capac1ty of said transformer to minimize the distortional effects on amplification at certain audio-frequencies caused thereby and to ensure a substantially uniform degree of amplification thruout the audio-frequency.

range. 1 a

7. In an audio-frequency amplifier, the combination with a pair of thermionic tubes each having anode, cathode and grid, of a two-winding unity ratio transformer coupling said tubes in cascade, with the primary winding terminal joined to the anode of the preceding tubehaving the same open-circuit polarity as the secondary winding terminal joined to the grid of the succeeding tube, a capacity connecting the anode of the preceding with the grid of/the succeeding tube, and a conductive path extending between said cathodes, said capacity being selectedon the basis of the leakage inductance and distributed capacity of the secondary winding of said transformer to minimize the distortional effects on amplification at certainw audio-frequencies. caused thereby and said transformer having unity ratio of transformation for minimizing dlstortional effects due to the flow of grid current in said secondary winding.

. ing terminal joined to the anode of the preceding tube having the same open-circuit clarity as the secondary winding terminal oined to the grid of the succeeding tube, a capacity connecting the anode of the preceding tube with the grid of the succeeding tube, and a conductive path extending between said cathodes, said capacity being selected on the basis of the leakage inductance and distributed capacity of the secondary winding of said transformer to minimize the distortional effects on amplification at certain audio-frequencies caused thereby, and said transformer having unity ratio of transformation for minimizing distortional effects due to the flow of grid current in said secondarly winding.

9. u an audio-frequency amplifier, the combination with a pair of thermionic tubes each having anode, cathode and grid, of a transformer having primary and secondary windings of minimized resistance, couplin said tubes in cascade, with the primary win ing terminal joined to the anode of the preceding tube having the same open-circuit polarity as the secondary winding terminal joined to the grid of the succeeding tube, a capacity extending from the anode of the preceding tube to the rid of the succeeding tube, and a 'COIldllCtlVG path extending between said cathodes said capacity being proportioned o-n the'basis of the distributed capacity and leakage inductance ofsaid secondary winding for minimizing the distortion'al efiects on amplification at certain audio-frequencies caused thereby, and said transformer being of substantially unity ratio for minimizing the distortional effects at audio-frequencies due to the flow of grid current in said secondary winding.

10. In an audio-frequency amplifier, the combination with a pair of thermionic tubes each having anode cathode and grid, of a two-winding transformer having its secondary winding connected to the input section of a first of said tubes, resistance serially included in circuit with said secondary winding and said input section,'and a capacity directly shunting the input section of said first tube, a two-winding transformencoupling said first and second tubes in cascade with the primary winding terminal joining the anode of the first tube having the same open-circuit polarity as the secondary winding terminal joining the grid of the second tube, a capacity connecting the anode of the first tube with the grid of the second tube and a conductive path extending between said cathodes, said aforementioned capacities and resistance being proportioned on the basis of the leakage inductances and distributed capacities of the Corresponding trans formers associated therewith to minimize the distortional effects on amplification caused thereby at certain audio-frequencies and to provide a substantially uniform degree of amplification over the audio-frequency range.

11. In an audio-frequency amplifier, the combination with a pair of thermionic tubes each having anode, cathode and grid, of a two-winding transformer having its secondary connected to the input section of a first of said tubes, resistance serially included in circuit with said secondary winding and said input section, and a capacity directly shunting the input section of said first tube, a two-winding transformer coupling said first and second tubes in cascade with the primary winding terminal joining the anode of the first tube having the same open-circuit polarity as the secondary winding terminal joining the grid of the second tube, a capacity connecting the anode of the first with the grid ofthe second tube-and a conductive ath extending between'said cathodes, said a orementioned capacities and resistance being proportioned on the basis of the leakage inductances and distributed capacities of the corresponding transformers associated there with to minimize the distortional effects .on amplification caused thereby at certain audiofrequencies and to provide a substantially uniform degree of amplification over the audio-frequency range, said transformer coupling said tubes in cascade having a ratio of transformation of substantially unity for minimizing the distortional eflects dueto the flow of grid current in the secondary winding of said transformer.

12. In an audio-frequency amplifier, the combination with a pair of thermionic tubes each having cathode and grid constituting an input section and an anode and said cathode constituting an output section, of a transformer coupling the output section of one said tube with the input section of the other, said transformer having a low transformation ratio in the order of one is to two as an upper limit selected to minimize the introduction of distortion into the amplifier due to the flow of grid current in the secondary winding of said transformer. Y

13. In an audio-frequency amplifier, the combination with a pair of thermionic tubes each having anode, cathode and grid, of a two winding transformer coupling the output section of one said tube with the input section of the other, said transformer having substantially unity ratio of transformation for minimizing distortional efi'ects introduced' into the amplifier due to the flow of grid current in the secondary winding of said transformer. 4

14. In an amplifier, thecombination with a thermionic tube having imput and output sections, of an input transformer associated with said input section, resistance external to said transformer serially included in circuit with said transformer and said input section, and a capacity directly shunting said input section, said resistance and capacity being so proportioned according to the leakage inductance and distributed capacity of said transformer as to insure a substantially uniform degree of amplification over a selected range in frequency.

15. In an audio-frequency amplifier, the

combination with a pair of thermionic tubes each having anode, cathode and grid, of a transformer having primary and secondary windings, said transformer serving to couple the output section of one said tube with the input section of the other, said transformer having a low ratio of transformation selected to approximately match the plate circuit'impedanceof the preceding tube with the grid circuit impedance of the succeeding tube when the grid thereof swings considerably positive, whereby the introduction of distortion due. to

the flow of grid current in said amplifier is minimized.

16. In an audio-frequency thermionic amplifier characterized by the absence of negative biasing potentials connected between cathode and input control electrode of the thermionic elements, the combination of a pair of thermionic tubes each having cathode and control electrode constituting an input section and an anode and said cathode constituting an output section, a transformer including primary and secondary windings coupling the output section of the one said section and an anode and said cathode constituting an output section, a transformer including primary and secondary windings coupling the output section of the-one said tube with the input section of the other, said transformer having a low transformation ratio in the order of one is to two as an upper limit, selected to minimize the introduction of distortion into the amplifier due to the flow of grid current therein.

18. In a low-frequency thermionic amplifier comprising a first tube and a second tube having a grid, in which the ratio of the out:

tube, causing the positive and negative aporput impedance of said first tube to the input impedance of said second tube is low when the grid of said second tube is charged cpnsiderably positively, said am lifier being 5 characterized by the absence 0 biasing potential on said grid of said second tube, the method of minimizing distortion which comprises transferring the signals from the output of the first tube to the input of the second tions of the transferred signal wave to ternately charge the grid of said second tube considerably positively and negatively in accordance with said signal wave portions, and

approximately matching the output impedance of said first tube with the input impedance of said second tube when said grid is charged considerably positively. V

.19. Ina low-frequency thermionic ampli- 29 fier comprising a pair of vacuum tubes each having an anode, cathode and grid, in which the ratio of the anode circuit impedance of the first of said tubes to the grid circuit impedance of the second of said tubes is low 2 when the grid of said second tube is charged considerably positively, said amplifier being characterized by the absence of negative biasing potential on the grid of said second tube, the method of minimizing distortion which 30 comprises transferring the signals from the anode circuit of the first of said tubes to the grid circuit of the second of said tubes, causing the grid of said second tube to be charged considerably positively by the transferred signals, and approximately matching the anode circuit impedance of said first tube with the grid circuit impedance of said second tube when the grid of said second tube is charged considerably positively. 4 In testimony whereof Iafiix my signature.

HAROLD ALDEN WHEELER. 

